Since version 2.39, Unison defaults to Unicode under Windows.
Add "unicode = false" to your profile if you need
backwards compatibility.

Upgrade tips (from Unison 2.39 or older):

make sure that your replicas are fully synchronized, either using
an older version of Unison or by temporarily turning off the Unicode
mode (unison -unicode false);

run Unison; it will complain that the archives are not in Unicode
mode; as a side-effects, it will also populate a fingerprint cache using the
archive contents; this cache is used to speed up the next step;

finally, run Unison once with the -ignorearchives option to
rebuild the archives.